One aspect of the invention relates to a method for local reduction of the operating noise produced by a medical diagnostic or therapy device, and medical diagnostic or therapy device having a device for implementing such a method.
Medical diagnostic or therapy devices, for example magnetic resonance tomographs or lithotripters, produce disturbing operating noise, to which the patient to be examined or treated is primarily exposed. High levels of noise occur for the patient, in particular in magnetic resonance tomographs, and, inside the magnet, can reach up to 120 dB(A) there. Such a noise level constitutes an enormous burden on the patient, both psychologically and physically, in particular under the restricted space conditions within a magnetic resonance tomograph. In order to reduce this burden, it is known to use, firstly, passive ear protection measures, for example earplugs or earmuffs, or, secondly, by constructive measures on the diagnostic and therapy device itself, to reduce its operating noise. Both measures are effective only up to a certain extent, since both passive ear protection and constructive measures are able to provide a remedy only to a certain extent. Passive hearing protection measures also have the disadvantage that these are problematic from a hygienic points of view and require the use of disposal articles.
In addition, a device with active ear protection is described in U.S. Pat. No. 5,427,102. Microphones are provided in the vicinity of the ears of the patient in order to pick up the noise produced by a magnetic resonance tomograph. Loudspeakers output what is known as antisound, which is determined in an electronic unit by the measured noise signals in such a way that the antisound largely compensates for the disturbing noise in the vicinity of the ears of the patient. In this case, the loudspeakers are either located in the immediate vicinity of the patient, or the antisound is led to the patient via acoustic waveguides in the form of tubes or pipes. In the first case, however, persons who are likewise present in order to operate the device and/or treat the patient sense the antisound as additional noise, since the antisound is not designed to match the noise level at the location of the operating/treating person. In the second case, the acoustic waveguides are laid loosely within the magnetic resonance tomograph. If the patient is moved, however, movement of the acoustic waveguides is also required. This is complicated and cumbersome. In addition, the frequency response of such an acoustic waveguide is limited, so that under certain circumstances it does not meet the requirements placed on the transmission of the antisound.
U.S. Pat. No. 5,313,945 also discloses a device for active noise compensation. Once again, acoustic waveguides are used, that can result in similar difficulties as described above.
In the case of the device disclosed by GB 2281970 A, the loudspeaker provided is an earphone with electric feedlines. Because of these feedlines, it is possible for the formation of surface waves to occur, which are hazardous to the patient and/or also distortion of the field from the high-frequency (HF) coils or antennas can occur. The λ/4 grounding provided for this reason cannot avoid the aforementioned hazards with absolute certainty, however. This is because the critical line wavelength substantially depends on the dielectric of the electric feedline. The exact value of the dielectric—which as a rule changes over time—is determined by many influencing variables. For example, the existing air space, the distance from the patient and also movements of the patient play a part.
EP 597528 A1 also describes a device for active noise compensation, in which headphones or loudspeakers, in particular with electric feedlines, are used. The question therefore arises again as to how the aforementioned hazardous surface waves and distortions of the HF field can be avoided.